1. Field of the Invention
This invention relates generally to semiconductor processing chambers, and relates more particularly in one embodiment to the wall structure of the chamber to improve structural integrity of the process chamber for low pressure applications.
2. Description of the Related Art
Process chambers for thermally processing semiconductor wafers typically are made of quartz (vitreous silica) or similar material because quartz is substantially transparent to radiant energy. Radiant heaters may be positioned adjacent the exterior of the chamber, and a wafer being processed in the chamber can be heated to elevated temperatures without having the chamber walls heated to the same level. Furthermore, quartz is desirable because it can withstand very high temperatures, and its inert characteristics enable it to withstand degradation by various processing gases.
For applications in which the pressure within a quartz chamber is to be reduced much lower than the surrounding ambient pressure, cylindrical or spherical chambers often are utilized because their curved surfaces are capable of withstanding the inwardly directed force. However, when positioning a flat wafer for chemical vapor deposition purposes where the deposition gases flow parallel to the wafer, it is desirable that the chamber walls be substantially parallel to the facing flat surface of the wafer, to obtain uniform deposition on the wafer surface. Uniform deposition is critical to obtain a high yield of acceptable products to be made from such wafer. However, a flat wall will collapse inwardly with reduced interior pressure sooner than will an outwardly convex wall of similar size and thickness.
To handle the inwardly directed forces on flat wall chambers, gussets or ribs have been provided on the exterior of the walls extending generally perpendicular to the wall to which they are joined, as may be seen in U.S. Pat. No. 4,920,918, issued on May 1, 1990, titled PRESSURE-RESISTANT THERMAL REACTOR SYSTEM FOR SEMICONDUCTOR PROCESSING, all of which is hereby incorporated by reference and made a part of this specification. One disadvantage of such a design is that even though quartz is substantially transparent to the radiant lamp energy, the rib sections present a region of much thicker quartz, and thus the ribs locally absorb more lamp energy and attenuate the lamp energy delivered to the wafer. This attenuation of energy causes cooler regions (i.e., shadows) on the wafer. Such non-uniformity of temperature on the wafer surface reduces the quality of the films that may be grown thereon, particularly for process conditions that are temperature-sensitive.
Temperature non-uniformities typically may be somewhat reduced by rotating the wafer during processing. Rotation causes regions of the wafer that would otherwise reside within shadows all of the time to pass into warmer regions, as well. Thus, rotation tends to distribute temperatures of the wafer surface somewhat more uniformly. It has been found, however, that if the ribs positioned forward of the wafer center (i.e., the axis of rotation of the wafer) are located at approximately the same distance from the wafer center as are the ribs positioned rearward of the wafer center, temperature uniformity suffers even with rotation. Specifically, forward and rearward ribs positioned at relatively similar distances from the wafer center cause a combined shadowing effect on the surface of the rotating wafer which can result in temperature distributions that are more non-uniform than can otherwise be achieved.